Method, apparatus and system for minimally intrusive fiber identification

ABSTRACT

A method, apparatus and system for minimally intrusive fiber identification includes imparting a time-varying modulation onto an optical signal propagating in an optical fiber and subsequently detecting the presence of the time-varying modulation in the optical signal transmitting through the fiber to identify the fiber. In a specific embodiment of the invention, a time-varying curvature is imposed on the fiber to be identified and the presence of the resultant time variation in the transmitted power of a propagating optical signal is subsequently detected for identification of the manipulated fiber.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.14/461,914, filed Aug. 18, 2014, which is currently allowed and is acontinuation of U.S. patent application Ser. No. 13/154,245, filed Jun.6, 2011, now U.S. Pat. No. 8,811,780, which is a continuation of U.S.patent application Ser. No. 11/870,684, filed Oct. 11, 2007, now U.S.Pat. No. 8,023,774, which is a continuation of U.S. patent applicationSer. No. 11/323,132, filed Dec. 30, 2005, now U.S. Pat. No. 7,283,688,which is a divisional of U.S. patent application Ser. No. 10/750,448,filed Dec. 31, 2003 and claims the benefit of U.S. ProvisionalApplication No. 60/496,448, filed Aug. 20, 2003, where the aboveidentified applications are all herein incorporated by reference intheir entirety.

FIELD OF THE INVENTION

The present invention relates generally to fiber identification and,more particularly, to a method, apparatus and system for identifyingfibers that minimally intrudes with optical signals propagating therein.

BACKGROUND OF THE INVENTION

Modern telecommunications offices have evolved in recent years toaccommodate greater volumes of traffic, thus placing larger and largeramounts of equipment (usually connected by optical fibers) in areas oflimited space. In addition to the increasing numbers of optical fibers,traffic carried by each of the optical fibers is also ever-increasing.As capacity increases, problems arise in the management of the opticalfibers of a telecommunication office. Specifically, because opticalfibers transport large amounts of high bit-rate traffic, the disruptionof such traffic leads to the disruption of service to many circuits and,as such, to many customers simultaneously.

For example, a typical telecommunications office includes a plurality ofracks of transmission equipment, each having multiple fiber connectionsto transmitters and receivers in the line cards supported in the racks.The fibers are ultimately destined for terminals either in that specificoffice, at customer locations, or in other offices. These office fibersare typically bundled and laid in fiber trays that provide paths orconduits to junction points such as patch panels (e.g., lightguide crossconnects) which connect the office fibers (sometimes called “jumpers”)to the outside plant (OSP) fibers which carry traffic from this officeto other destinations. Over time, the exact connection paths (i.e., theconnection paths between ports on a lightguide cross connect tocorresponding ports on the line cards in the racks) may become unknowndue to, for example, labels used to identify fibers falling off), fibersbeing initially labeled incorrectly, or emergency maintenance actionrequiring a fast response not being properly documented. Theunidentified or mis-identified fiber connections can ultimately lead todisastrous Quality of Service conditions. For example, assume that atechnician, in the course of responding to a (loss of light) alarm,disconnects a fiber labeled as being connected to a port identified asthe source of the alarm. If the fiber connection is mislabeled orunknown, the technician may in fact be disrupting a properly functioningcircuit, thus creating a new error and disruption of service anddelaying the repair of the original faulted circuit. As such, severalmeans have been proposed for identifying a fiber without interruptingtraffic on the fiber connection.

Such proposed means for the identification of optical communicationcircuits include Local Injection (LI) and Local Detection (LD) methodsthat have been used in practice for fusion splicing. These techniquesinvolve bending a bundle of optical fibers in a cable at two distantlocations and injecting light into the fiber at one bent portion whiledetecting the injected light that leaks from the fiber at the other bentportion. This method however, has several disadvantages. For example, inorder to inject an adequate amount (i.e., power) of light into thecoated fiber to be later detected, the fiber must be bent with acurvature large enough (i.e., radius of curvature small enough) toinject light thus causing radiated light of a large power to leak fromthe bent portion of the fiber to which the LI method is to be applied.This causes deterioration of a signal that is to be transmitted by thebent fiber. Therefore, if the LI method is applied during transmissionof an optical signal, troubles such as channel interruption will occurin optical signal communication, and in an extreme case, cracking mightoccur in the coated fiber. In addition, if light having a power greaterthan a threshold level is injected into a fiber by the LI method, theinjected light may be transmitted to an office or to subscribersresulting in the addition of a noise component that may deteriorate anoptical signal being transmitted.

Therefore, a need exists for a method and apparatus for theidentification of optical fibers that minimally intrudes with opticalsignals propagating therein.

SUMMARY OF THE INVENTION

The present invention provides methods and apparatuses for fiberidentification that minimally intrudes with a propagating optical signaltherein.

In one embodiment of the present invention, a method includes varying aproperty of an optical fiber as a function of time such that atime-varying modulation is imparted on an optical signal propagatingtherein, and subsequently detecting the time-varying modulation toidentify the optical fiber. More specifically, in one embodiment of thepresent invention, a curvature of at least a portion of an optical fiberis varied as a function of time such that a small time-varying loss ofpower is generated in the propagating optical signal. The time-varyingloss of power is subsequently detected downstream to unambiguouslyidentify the optical fiber.

In an alternate embodiment of the present invention, birefringence of anoptical fiber is varied as a function of time such that the polarizationof an optical signal propagating therein is varied as a function oftime. A detector adapted for the detection of the time-variedpolarization (i.e., a detector including a polarizer) subsequentlydetects the time-varying polarization to identify the optical fiber.

In yet another embodiment of the present invention, a property, such asthe frequency, of an optical signal propagating in an optical fiber isvaried as a function of time. The time-varying altered property (e.g.,the frequency of the optical signal) is subsequently detected foridentification of an optical fiber transmitting the optical signal.

BRIEF DESCRIPTION OF THE DRAWINGS

The teaching of the present invention can be readily understood byconsidering the following detailed description in conjunction with theaccompanying drawings, in which:

FIG. 1 depicts a high level block diagram of a telecommunications officewherein an embodiment of the present invention may be applied;

FIG. 2 depicts a high level block diagram of an embodiment of a fiberidentification device of the present invention;

FIG. 3 depicts a high level block diagram of an embodiment of a fiberidentification system in accordance with the present invention;

FIG. 4 depicts a high-level block diagram of an embodiment of a controlunit suitable for use in the fiber identification device of FIG. 2 andthe fiber identification system of FIG. 3;

FIG. 5 depicts a high level block diagram of an alternate embodiment ofa fiber identification device of the present invention; and

FIG. 6 depicts a high level block diagram of a passive optical network(PON) including an embodiment of a fiber identification device of thepresent invention.

To facilitate understanding, identical reference numerals have beenused, where possible, to designate identical elements that are common tothe figures.

DETAILED DESCRIPTION

Although various embodiments of the present invention herein are beingdescribed with respect to optical fibers within a telecommunicationsoffice, it should be noted that the optical fibers and thetelecommunications office presented herein are simply provided asexemplary working environments wherein various embodiments of thepresent invention may be applied and should not be treated as limitingthe scope of the invention. It will be appreciated by those skilled inthe art informed by the teachings of the present invention that theconcepts of the present invention may be applied to a single ormultiply-interconnected optical fibers (or waveguides) in substantiallyany working environment (local or remote) for the identification of thetransmission medium.

FIG. 1 depicts a high level block diagram of a telecommunications officewherein an embodiment of the present invention may be applied. Thetelecommunications office 100 of FIG. 1 illustratively comprises threeracks of transmission equipment 102, 103, and 104, each rack having,connected to line cards therein (not shown), the first end of aplurality of transmission fibers 105 ₁-105 _(n), 110 ₁-110 _(n), and 115₁-115 _(n), respectively. The telecommunications office 100 furthercomprises a plurality of cable trays 120, and a patch panel(illustratively a lightguide cross connect (LGX)) 125. Second ends ofthe plurality of transmission fibers 105, 110 and 115 are connected toports on a first side of the LGX 125. A plurality of outside plantcables 130 ₁-130 _(m) is connected to ports on a second side of the LGX125. The telecommunications office 100 further comprises an embodimentof a fiber identification device 140 in accordance with the presentinvention. In FIG. 1, a first portion of the fiber identification device140 of the present invention is illustratively located on a bottom fiberof the transmission rack 104 and a second portion of the fiberidentification device 140 is located on a bottom fiber of the first sideof the LGX 125.

In the telecommunications office 100, at least some of the plurality offibers 105 may interconnect the various ports of the line cards (notshown) of the transmission rack 102 to ports on the first side of theLGX 125 (for traffic that is ultimately destined to go outside theoffice, for example on an outside plant fiber (OSP)). Similarly, atleast some of the plurality of fibers 110 and 115 may interconnect thevarious ports of the line cards (not shown) of the transmissionequipment racks 103 and 104, respectively, to ports on the first side ofthe LGX 125. In operation, communication between the transmission racks102, 103 and 104 and the LGX 125 is accomplished over the transmissionfibers 105, 110 and 115, respectively. An application of the fiberidentification device 140 of the present invention is used to identifyto which port of the LGX 125 a particular fiber from one of thetransmission racks 102, 103 and 104 is connected or vice-versa. Forexample, in FIG. 1 the fiber identification device 140 of the presentinvention is connected to a bottom fiber of the transmission rack 104and to a bottom fiber of the first side of the LGX 125 to determine ifthe fiber connected to a bottom port of the transmission rack 104 is thesame fiber as the fiber connected to a bottom port of the first side ofthe LGX 125.

In accordance with the aspects of the present invention, an opticalfiber is identified by imparting a time-varying modulation on an opticalsignal propagating in the optical fiber and subsequently detecting thepresence of the imparted time-varying modulation to identify the opticalfiber. In the present invention, the imparted time-varying modulationand the subsequent detection are performed such that the propagation ofthe optical signal in the optical fiber is not interrupted (described ingreater detail below). For example, in various embodiments of thepresent invention an optical fiber is identified by varying a propertyof the optical fiber as a function of time, and then detecting thepresence of the imparted variation in an optical signal propagatingthrough that fiber that is correlated to the variation of the opticalfiber property. For example, the curvature of an optical fiber may bevaried as a function of time to impart a time-varying loss of power inan optical signal propagating therein. The presence of the impartedtime-varying loss of power is then subsequently detected to identify thesubject fiber.

In alternate embodiments of the present invention, an optical signal isidentified by imparting a time-varying modulation on an optical signaloutside of the fiber in which it is being transmitted. For example, atime-varying modulation may be imparted on an optical signal at anintermediate point between two sections of fiber (i.e., at the locationof a free space beam expander), and the presence of the modulationsubsequently identified in the transmission fiber to identify the fiberpath. In addition, a time-varying modulation may be imparted on one or aplurality of optical signals outside of a fiber or fiber path in whichthe optical signals are to be transmitted. For example, a transmitter ortransmitters may be controlled to apply a distinctive additional signalor over-modulation (i.e., time-varying modulation) of an optical signalto be transmitted in an optical fiber or fibers to be identified.

Finally, it should be noted that in the description of the variousembodiments herein, the use of the term “fiber” may be used to identifya single fiber for transmitting an optical signal, or a fiber pathcomprising a plurality of interconnected fibers for transmitting anoptical signal across a network. That is, the aspects of the presentinvention may be implemented to identify a single fiber transmitting anoptical signal, or to identify an optical path on a network of aplurality of interconnected fibers possibly carrying a plurality ofoptical signals. The time-varying modulation may be imparted on one or aplurality of optical signals by varying a property of an optical fiberpropagating the optical signal. In addition a time-varying modulationmay be imparted on one or a plurality of optical signals outside of afiber or fiber path in which the optical signal is being transmitted(i.e., time-varying modulation imparted at an intermediate point betweeninterconnected fibers).

More specifically, a detectable unique signature is imparted on opticalsignals propagating through a subject optical fiber or optical fiberpath and the imparted signature is subsequently detected to identify thesubject optical fiber or path. The inventors herein illustrativelydepict three properties of light, namely; polarization (i.e., thedirection of the oscillating electric field); frequency; and amplitude(the electric field strength) or power (proportional to its square);that may be used in the implementation of the invention disclosedherein. The manipulation of the aforementioned three properties oflight, which are used to impose a detectable “signature” on thetransmitted light which is subsequently used to identify an opticalfiber, are discussed in greater detail below. Although variousembodiments of the present invention are being described herein asmanipulating three optical properties for imparting a detectablesignature on an optical signal propagating in an optical fiber, it willbe appreciated by those skilled in the art informed by the teachings ofthe present invention that more seemingly sophisticated forms ofimparting a time-varying modulation on an optical signal propagating inan optical fiber (e.g., phase modulation) and more seeminglysophisticated forms of detection (e.g., heterodyne detection) may beimplemented within the concepts of the present invention to identify asubject optical fiber or fiber path.

FIG. 2 depicts a high level block diagram of an embodiment of a fiberidentification device of the present invention suitable for use in thetelecommunications office of FIG. 1. The fiber identification device 140of FIG. 2 illustratively comprises a lightguide (illustratively aplexiglass lightguide) 210, a fiber bending device (illustratively aclamping anvil) 220, a detector (illustratively a photodiode) 230, and amodulating device (illustratively a vibrating piston) 240. FIG. 2further depicts an optical fiber 250 inter-positioned between theclamping anvil 220 and the plexiglass lightguide 210 to illustrate theconcepts of the present invention. For convenience, the embodiment ofFIG. 2 depicts an embodiment of the present invention which combines theaspects of modulation and detection processes in a single device.

In the fiber identification device 140 of FIG. 2, the vibrating piston240, the clamping anvil 220 and the photodiode 230 are depicted as beingin close proximity for illustrative purposes and for ease ofexplanation. It will be appreciated by those skilled in the art informedby the teachings of the present invention that a vibrating piston, aclamping anvil and a photodiode of the present invention may be as closeor as far apart as necessary to perform the identification method of thepresent invention as dictated by economics of production. Morespecifically, a fiber identification device in accordance with thepresent invention may be comprised of a transmitter head (i.e.,comprised of the vibrating piston 240 of FIG. 2) and a receiver head 225(i.e., comprised of the lightguide 210, the clamping anvil 220 and thedetector 230. The transmitter head 240 may be located on a fiber within,for example, a telecommunications office for imparting a time-varyingmodulation on an optical signal propagating in the optical fibers of thetelecommunications office while the receiver head 225 may be locatedhundreds of meters or kilometers away for detecting the presence of theimparted time-varying modulation for identification of a subjectfiber(s).

In addition, although the vibrating piston 240 of FIG. 2 is depicted asoperating on a bent portion of the subject fiber, in alternateembodiments of the present invention wherein a transmitter head and areceiver head are located distances apart from each other, a vibratingpiston may impart a time-varying modulation on an optical signal byvibrating a substantially straighter portion of the optical fiber toincrease (by adding curvature) or decrease (by reducing curvature) thebending loss applied to an optical fiber. Furthermore, although in FIG.2, the modulating device is depicted as a vibrating piston, it will beappreciated by those skilled in the art informed by the teachings of thepresent invention that the modulating device may be any such devicecapable of providing a mechanical vibration or biasing curvature of thefiber. Even further, the modulating device in other embodiments of thepresent invention may be substantially any component capable ofimparting a time-varying modulation on an optical signal propagating insubject optical fibers as described throughout this disclosure.

Furthermore, although in FIG. 2, the vibrating piston 240, the clampinganvil 220 and the photodiode 230 are depicted as comprising separatecomponents, in alternate embodiments of the present invention, thevibrating piston, the clamping anvil and the photodiode of the presentinvention may comprise a single component, multiple components orsubstantially any combination thereof.

Referring back to FIG. 2, in the fiber identification device 140 thevibrating piston 240 is mechanically driven such that it vibrates thefiber 250 and produces a time-varying curvature of the fiber 250, andthus a time-varying loss (i.e., a power variation) in an optical signalguided by the fiber 250 due to “bending loss”. Further downstream, afiber to be tested for identification (illustratively the fiber 250) isbent by the clamping anvil 220. The basic principle is that when thefiber 250 is bent, some of the light (i.e., traffic on the fiber oralternatively, a test signal) is scattered out of the fiber core andsubsequently out of the fiber 250, itself. As such and as illustrated bythe smaller graphs on the lower left, the lower right, and on the topright of FIG. 2, the power carried by the fiber's fundamental mode isessentially divided into two components. A first component representsthe remaining signal in the fiber 250 (graph on lower right) andcontinues propagating along the fiber 250. The second componentrepresents the portion of the signal that has been scattered out of thefiber 250 (graph on upper right). At least a portion of this scatteredsignal is collected by the plexiglass lightguide 210 and guided to thedetector 230. The detector 230 is configured to have electronicssensitive to the corresponding frequency of the time-varying lossimparted by the vibrating piston 240. As illustrated in FIG. 2, thelight leaving the fiber has a small ac component to its amplitude(dithered) due to the overmodulation of the signal propagating in thefiber 250 imparted by the vibrating piston 240 and the detector 230 iswell suited to detect power variations at the overmodulationfrequencies. Although in FIG. 2 the fiber identification device 140, isillustratively depicted as comprising a lightguide for guiding thescattered light out of the fiber 250, in alternate embodiments of thepresent invention, a fiber identification device of the presentinvention does not comprise a lightguide and, as such, the lightscattered from a bent fiber is detected directly by an included detectorinstead.

In accordance with the present invention, the frequency of the dither(i.e., the frequency of the vibration imparted by the vibrating piston240) is well established and the dither amplitude and static biasingcurvature is chosen such that the dither amplitude is small compared tothe average power of the signal propagating in the fiber 250 in order tominimally impact the transmitted signal. The detector 230 needs to onlysensitively detect the presence of the time variation at the frequencyimparted by the vibrating piston 240. The detection of the consistentpresence of the time-varying signal at the imparted frequency by thedetector 230 is a clear indication that the vibrating piston 240 isacting on that particular fiber upstream and as such, the fiber 250 isidentified. That is, if the vibrating piston 240 were not actingupstream on the particular fiber now being tested, the detector 230would only detect random noise within a detection bandwidth centeredabout the frequency of the modulation (dithering) imparted by thevibrating piston 240. While there would be some spectral content at thecentral frequency of the detector 230 in the scattered signal of a fiberthat is not being dithered, it would be quite distinguishable from thedithered signal.

In the detection method of the present invention, there is little needfor accurate calibration. That is, it is sufficient to detect thepresence of the fundamental frequency imparted by the vibrating piston240 rather than its exact amplitude. Furthermore, the detection of thepresent invention is an AC measurement, which may be performed with muchhigher gain, discrimination, and lack of bias drift when compared to anequivalent DC measurement. Even further, in various embodiments of thepresent invention the most pertinent information carried by thescattered, dithered signal is the dithering frequency and its presence.As such, the filter bandwidth of a detector, such as the detector 230 ofFIG. 2, may be restricted to a very narrow range (i.e., the detectiontime constant may be on the order of seconds) since it is only necessaryto confirm the presence of the dithering frequency. Thus, the detectionof the present invention may be quite sensitive. In short, theidentification process is essentially complementary, in a measurementsense, to an accurate power measurement.

A fiber identification device in accordance with the present inventionmay further comprise a control unit. For example, the fiberidentification device 140 of FIG. 2 further comprises a control unit 275to enhance the operation when the transmitter head 240 is remotelylocated from the receiver head 225. The control unit 275 is adapted tocontrol the transmitter head 240 and/or the receiver head 225. Forexample, the transmitter head 240 may be placed on a portion of thefiber 250 by a technician. The technician may subsequently place thereceiver head 225 downstream on the fiber 250 at a position on the fiber250 located hundreds of meters or kilometers away. After placing thereceiver head 225 on the fiber 250, the technician may then remotelysend a signal (e.g., a radio or Ethernet signal) to the control unit 275to cause the control unit 275 to generate a control signal to begin thetime-varying modulation of the optical signal in the optical fiber 250by starting the operation of the vibrating piston 240. The control unit275 may also be configured to be capable of adjusting the amplitude (orfrequency) of the vibration of the vibrating piston 240 (or biasingcurvature) to optimize the fiber identification device 140 of thepresent invention by choosing an amplitude (or frequency) of vibrationthat will minimally intrude with a propagating optical signal in thefiber 250. Although in the description directly preceding it isdescribed that a remote signal is sent to the control unit to generate acontrol signal to initiate and control the operation and function of thetransmitter head 240, a control unit in accordance with the presentinvention may either be hard wired to one or both, the transmitterhead(s) and the receiver head(s), for communication or may alternativelybe in communication with one or both, the transmitter head(s) and thereceiver head(s), via remote means.

Furthermore, in an alternate embodiment of the present invention, afiber identification device may comprise more than one transmitter head.As such, a technician may place a transmitter head on each of aplurality of fibers in, for example, a telecommunications office andthen head out to the field. The technician may then test the fibers at aposition downstream located on the fiber hundreds of meters orkilometers away by placing a receiver head on the fibers one at a time.The technician may send a remote signal (e.g., a radio signal) to anincluded control unit adapted to turn one of the transmitter heads on ata time while placing the receiver head on the fibers one at a time todetect the presence of the time-varying modulation imparted on arespective optical signal to identify the optical fiber associated withthe transmitter head that is on. In this manner, a plurality of fibersmay be identified remotely by a technician. Alternatively, more than oneof the plurality of transmitter heads may be turned on at once, each ofthe transmitter heads having a different frequency of vibrationassociated with it, and as such, the receiver head may be used toidentify the presence of the various known frequencies to identify anoptical fiber associated with the transmitter head vibrating at aspecific known frequency. Although in the description above variousembodiments of the fiber identification device of the present inventionare depicted and described as having a remotely controlled transmitterhead(s), a receiver head(s) may also be remotely controlled by sending aremote signal (e.g., a radio signal) to an included controller foroperatively controlling the receiver head(s) (e.g., the detector(s) andany adjustable bending devices) of the present invention.

Even further, in yet another embodiment of the present invention, agroup of fiber identification devices in accordance with the presentinvention may comprise a fiber identification system. That is, a fiberidentification system of the present invention may comprise more thanone transmitter head and more than one receiver head. For example, FIG.3 depicts a high level block diagram of an embodiment of a fiberidentification system 300 in accordance with the present invention. Thefiber identification system 300 of FIG. 3 comprises a plurality oftransmitter heads 340 ₁-340 _(N) (collectively transmitter heads 340), aplurality of receiver heads 325 ₁-325 _(N) (collectively receiver heads325), a plurality of optical fibers 350 ₁-350 _(N) (collectively opticalfibers 350) and a controller 275. As in the fiber identification device140 of FIG. 2, each of the transmitter heads 340 of the fiberidentification system 300 includes at least one modulating device, suchas a vibrating piston (not shown). Similarly, as in the fiberidentification device 140 of FIG. 2, each of the receiver heads 325 ofthe fiber identification system 300 of FIG. 3 includes at least a fiberbending device (e.g., an anvil (not shown)) and a detector (not shown).Alternatively, the receiver heads 325 of the fiber identification system300 of FIG. 3 may each further include a lightguide.

The plurality of transmitter heads 340 and the plurality of receiverheads 325 in the fiber identification system 300 of FIG. 3 may be placedon respective fibers by a technician in an attempt to identify specificoptical fibers. More specifically, a technician may connect atransmitter head 340 on a position on each of the plurality of opticalfibers 350 that may be located in, for example, a telecommunicationsoffice. The technician may then head out to the field. The technicianmay then connect a receiver head 325 downstream on each of the pluralityof fibers 350 thought to be the same fibers or in the same fiber pathsas the fibers in the office at a position located hundreds of meters orkilometers away. In such embodiments, the fiber identification method ofthe present invention may be executed manually or automatically. Forexample, a technician may send a remote signal (e.g., a radio signal) tothe control unit 275 which is adapted to control the transmitter heads340 for turning the vibration of the transmitter heads 340 on and off.The technician may then choose to monitor different ones of theplurality of receiver heads 325 to identify the presence of thetime-varying modulation imparted by the specific ones of the transmitterheads 340 to identify specific fibers.

Alternatively and for automatic operation, the control unit 275 of thepresent invention may be adapted to automatically control the operationof the plurality of transmitter heads 340 and the plurality of receiverheads 325 in substantially any combination and frequency to identify thesubject optical fibers by iteratively controlling respective ones of thetransmitter heads 340 and the receiver heads 325 to identify thepresence of an imparted respective time-varying modulation imparted onrespective propagating optical signals and as such, identify each of theplurality of optical fibers. The control unit 275 of the presentinvention keeps track of which transmitter heads 340 are operating andat what frequencies. Each of the plurality of receiver heads 325transmits the respective outputs of the detectors (not shown) of thereceiver heads 325 to the control unit 275. The control unit 275,knowing which detected signal was received from which receiver head 325,is able to identify the plurality of optical fibers 350 by examining thereceived outputs of the receiver heads 325 and identifying therespective time-varying modulation imparted on the respective opticalsignals propagating in the respective optical fibers 350.

Alternatively, the fiber identification system 300 of FIG. 3 may beconfigured to comprise a single detector for receiving the scatteredportions of the respective optical signals from the respective opticalfibers. In such an embodiment, the single detector is configured suchthat it is operative to receive respective scattered portions of theoptical signals of the respective optical fibers one at a time and tosend the respective detected information to the control unit. Thecontrol unit may then identify the respective optical fibers from theinformation received from the single detector.

Even further, in various embodiments of the present invention, a controlunit of the present invention may be adapted to generate a controlsignal to cause a signal source (i.e., a signal transmitter) to apply adistinctive additional signal or over-modulation (i.e., time-varyingmodulation) of an optical signal to be transmitted in an optical fiberor fibers to be identified. That is, a control unit of the presentinvention may be adapted to control a modulator of one or moretransmitters of a system to cause a time-varying modulation to beimparted by the transmitters on respective optical signals to betransmitted on respective optical fibers for subsequent identificationof the optical fibers in accordance with the present invention.

FIG. 4 depicts a high-level block diagram of an embodiment of a controlunit suitable for use in the fiber identification device 140 of FIG. 2and the fiber identification system 300 of FIG. 3. The control unit 275of FIG. 4 comprises a processor 410 as well as a memory 420 for storinginformation and control programs. The processor 410 cooperates withconventional support circuitry 430 such as power supplies, clockcircuits, cache memory and the like as well as circuits that assist inexecuting the software routines stored in the memory 420. As such, it iscontemplated that some of the process steps discussed herein as softwareprocesses may be implemented within hardware, for example, as circuitrythat cooperates with the processor 410 to perform various steps. Thecontrol unit 275 also contains input-output circuitry 440 (i.e., may beremote input-output circuitry) that forms an interface between thevarious functional elements communicating with the control unit 275. Forexample, in the embodiment of FIG. 2, the control unit 275 communicateswith the transmitter head 240 via a signal path S1 and to the receiverhead 225 via signal path O₁.

Although the control unit 275 of FIG. 4 is depicted as a general purposecomputer that is programmed to perform various control functions inaccordance with the present invention, the invention can be implementedin hardware, for example, as an application specified integrated circuit(ASIC). As such, the process steps described herein are intended to bebroadly interpreted as being equivalently performed by software,hardware, or a combination thereof.

Although in the embodiments of fiber identification devices and a fiberidentification system in accordance with the present invention depictedabove (e.g., FIG. 2 and FIG. 3) the modulating device is depicted ascomprising a vibrating piston 240, 340, various other means for imparteda time-varying modulation (e.g., time-varying loss) on an optical signalpropagating along on optical fiber, such as piezo-electric transducers,motors and vibrators, may be implemented in a fiber identificationdevice and a fiber identification system in accordance with the presentinvention. More specifically, the bend loss imparted in a fiber may becharacterized according to equation one (1), which follows:α=c ₂exp(−c ₁ R),  (1)where α is the imparted loss per unit length (and can be considered asproportional to the light scattered into the detector), R is the fiber'sradius of curvature, and c₂ and c₁ are constants which are not strongfunctions of R, but are functions of the fiber design and the wavelengthof light propagating in the fiber. The exponential dependence on Rshould be noted. More specifically it should be noted that as Rdecreases, the scattered light goes from very small values to very largevalues quite rapidly. As a consequence of this exponential dependence, agiven curvature (static or biasing) at the location of the vibratingpiston may be imparted, such that small additional changes applied makemore significant variations in the loss without the necessity of makinglarge variations in the bend radius of the fiber. Because of thedependence of c₁ and c₂ on fiber types and wavelengths, the implicationis that fiber identification devices of the present invention mayimplement a variable fiber bending device (e.g. a variable anvil, ormore than one anvil) to apply the bends at both the location of thevibrating piston and the location of the anvil and detector.

Therefore, to exploit the advantages of the present invention whileminimizing the system impact (recalling that the loss may vary stronglywith wavelength), various embodiments of the present invention comprisea means for providing an adjustable bending radius to optical fibers andthus, adjustable bending losses for varied applications. For instance,an embodiment of a fiber identification device of the present inventionmay comprise various fiber bending devices (e.g., anvils), eachpossessing a different radius of curvature. As such, when an accurate DCmeasurement must be made on a fiber wherein loss is not too great anissue, a smaller radius anvil would be implemented. The smaller radiusanvil would impose more loss on the fiber, but would insure accuracy bydiverting more power to the detector. Such a method is preferable forapplications in tracing, characterizing, and inventorying dark fibers,for example, where loss is not an issue.

On the other hand, for applications in which loss is an issue, such asfibers carrying high speed traffic that operate near the margin of theirpower budgets, it is imperative to minimize loss. In such applications,the modulation frequency, for example from the vibrating piston, iswell-known and thus can be easily detected, while its magnitude may notnecessarily need to be accurately ascertained. Thus, to insure minimalloss, the radius of curvature of the fiber bending device would belarger in order to apply less loss to a propagating optical signal inthe fiber, and thus minimizing the time-variant component of loss. Thisaspect of the present invention may be implemented by providing a set ofinterchangeable fiber bending devices or by implementing adjustablefiber bending devices that increase or decrease the radius of curvatureof the fiber in the active region via an adjustment. Various other meansfor providing adjustable bending radii for optical fibers, such assliding clamps, levers, detents, etc., are known and such other meansmay be implemented in a fiber identification device in accordance withthe present invention.

In addition, because the amount of bend loss experienced by an opticalsignal depends on the wavelength of an optical signal or the wavelengthsof an optical signal (i.e., WDM signals are comprised of variouswavelengths), with the longer wavelengths being more lossy than theshorter wavelengths, a bend radius for scattering light out of anoptical fiber in accordance with the present invention must be carefullychosen. More specifically, a bend radius that causes a negligible losson a short wavelength might cause a severe loss on a much longerwavelength. For instance, in current coarse WDM (CWDM) systems, it iscommon for there to be a 140 nm separation between the top and thebottom wavelengths, and as such, there can be significant differences inthe bend losses that may result in catastrophic losses on the longerwavelengths.

In the present invention, any vibration that is applied to a fiber wouldpreferably be applied in such a way that it does not damage the fiber.For example it may be advantageous to configure a subject fiber to havea freestanding section to which a vibration in accordance with thepresent invention is applied. As such, the fiber would be lesssusceptible to damage from the modulation. Furthermore and with regardto the acoustic frequency of operation, the conventional frequencychoices (e.g., 270, 1000, 2000 Hz) to commercial live fiber indicatorsare resistant to light leaks from office lights, etc., at 60 Hz and 120Hz. However, since the energy of a vibrating fiber is proportional tothe square of its velocity, the power consumption of the bending device,such as the piston, will scale substantially with the square of thefrequency. Accordingly, it may be advantageous to set the frequency tobe even lower than 270 Hz while recognizing the prevalence of the 60 Hznoise sources.

In typical prior art fiber identification devices, AC tones used toidentify fibers are essentially modulated at 100% depth (i.e., an on-offmodulation at acoustic frequencies) and are implemented on “dark fibers”carrying no traffic on live fiber. Such deep modulation, however, surelyimpairs the signal transmission. That is, the light lost in impressingthe signature tone will manifest itself as an additional time-varyingloss, closing the “eye pattern” (as familiar to those skilled in theart) by that same amount and would completely close the eye pattern fordeep modulation. Thus, it would be preferable to have only a smallmodulation depth (i.e., loss corresponding to less than a dB loss, forexample) on the signal light. As such, in various embodiments of thepresent invention, the light corresponding to the logical “1s” whichleave a subject fiber after experiencing the imparted vibrating bendloss, do not vary between 0 and 1 as they do for 100% modulation, butpreferably vary between levels which are close to 1, such as between0.85 and 0.95 of the original amplitude. In such an example, the fiberidentification device of the present invention would perceive a 5%insertion loss and a 10% time-dependent loss, which, as far as systemperformance is concerned, is considered a 15% loss, or approximately 0.7dB.

FIG. 5 depicts a high level block diagram of an alternate embodiment ofa fiber identification device of the present invention. The fiberidentification device 500 of FIG. 5 comprises substantially the samecomponents as the fiber identification device 140 of FIG. 2 with theaddition of a second lightguide and a second detector. The fiberidentification device 500 of FIG. 5 illustratively comprises a first anda second lightguide (illustratively plexiglass lightguides) 510 ₁ and510 ₂, a clamping anvil 520, a first and a second detector 530 ₁ and 530₂ (collectively detectors 530), and a modulating device (illustrativelya vibrating piston) 540. In the fiber identification device 500 of FIG.5, the first lightguide 510 ₁, the vibrating piston 540 and the firstdetector 530 ₁ comprise a transmitter head 502, and the secondlightguide 510 ₂, the clamping anvil 520 and the second detector 530 ₂comprise a receiver head 504. FIG. 5 further illustrates an opticalfiber 550.

Similar to the fiber identification device 140 of FIG. 2, in the fiberidentification device 500 of FIG. 5, the vibrating piston 540 ismechanically driven such that it vibrates the fiber 550 and produces atime-varying curvature of the fiber 550, and thus a time-varying loss(power variation) in the fiber 550 due to “bending loss”. The lossimparted by the vibrating piston 540 causes some light to scatter out ofthe fiber. In the fiber identification device 500 of FIG. 5, the firstplexiglass lightguide 510 ₁ collects at least a portion of the lightscattered due to the vibration of the optical fiber 550 and guides thescattered light to the first detector 530 ₁. The first detector 530 ₁detects the scattered light from the first plexiglass lightguide 510 ₁.The light from the first plexiglass lightguide 510 ₁ and detected by thefirst detector 530 ₁ may be used to verify the presence and direction oflight in the optical fiber 550, that the vibrating piston 540 isoperating correctly (i.e., that the light in the optical fiber does infact have an imparted time-varying modulation), and/or to provide areference signal to compare to the time-varying modulation subsequentlydetected by the second detector 530 ₂. Alternatively, the scatteredlight detected by the first detector 530 ₁ may be used to identify howmuch power is being scattered out of the fiber due to the vibrationwhich may be used to adjust the amplitude and the frequency of thevibration of the vibrating piston 540 to optimize the fiberidentification device of the present invention such that a propagatingoptical signal is minimally affected.

Further downstream, the fiber to be tested for identification(presumptively the fiber 550 or a fiber connected to it) is bent by theclamping anvil 520. As previously described, when the fiber is bent,some of the light is scattered out of the fiber. At least a portion ofthis scattered signal is collected by the plexiglass lightguide 510 ₂and is guided to the second detector 530 ₂. As illustrated in FIG. 5 andas previously described, the light leaving the fiber 550 has a small ACcomponent to its amplitude (dithered) due to the overmodulation of thesignal propagating in the fiber 550 imparted by the vibrating piston540.

As before, the second detector 530 ₂ needs to detect only the presenceof the time variation at the specific frequency imparted by thevibrating piston 540. The detection of the presence of the time-varyingsignal at the imparted frequency by the second detector 530 ₂ enablesthe identification of the fiber. As previously stated, the additionaldetector 530 ₁ enables the detection and measurement of the time-varyingmodulation (i.e., signature) imparted on a propagating optical signal,the signature to be later detected for identifying a subject opticalfiber.

In alternate embodiments of the present invention, the transmitter head502 of the fiber identification device 500 of FIG. 5 further comprises aclamping anvil to apply a biasing curvature and the receiver head 504further comprises a modulating device (e.g., a vibrating piston) suchthat the fiber identification device of the present invention is capableof being used in either direction to identify an optical fiber. That is,in a fiber identification device in accordance with this embodiment, anoptical fiber located in the region of a receiver head may be modulatedsuch that a time-varying modulation is imparted on an optical signalpropagating therein. The modulated optical fiber may then be identifiedupstream by a transmitter head of the fiber identification device inaccordance with this embodiment. Alternatively, an optical fiber locatedin the region of the transmitter head may be modulated such that atime-varying modulation is imparted on an optical signal propagatingtherein. The modulated optical fiber may then be identified downstreamby the receiver head of the fiber identification device in accordancewith this embodiment

In alternate embodiments of the present invention, the polarization oflight propagating in an optical fiber may be manipulated to produce atime-varying modulation that may later be detected to identify theoptical fiber. For example, the polarization of light may be modified bydisturbing the normal propagation of light in some way, introducingbirefringence in the fiber (i.e., changes to the index of refractionwhich are non-uniform) by altering the fiber in some way, or by applyinga field that couples to a property in the fiber. For illustrativepurposes the Faraday effect, caused by magnetic fields, is considered.For example, when an axial magnetic field is applied to a fiber carryingan optical signal, it makes light with right circular polarizationtravel at a different velocity than light with left circularpolarization. More specifically, a circular birefringence is imparted.For linearly polarized light, applying an axial magnetic field to afiber carrying an optical signal thereby makes the direction ofpolarization change. Various means for applying a magnetic field, suchas a solenoid, to a fiber carrying an optical signal to produce atime-varying modulation in the optical signal may be implemented withinthe concepts of the present invention. The inventors further considerand propose that the application of an electric field will have asimilar effect as the application of a varying magnetic field asdescribed above. As such, various means for applying an electric fieldto a fiber carrying an optical signal to produce a time-varyingmodulation in the optical signal, such as by adapting an opticalfiber(s) to comprise electrodes, may be implemented within the conceptsof the present invention.

For example, and referring to FIGS. 1, 2, 3 and 5 above, a time-varyingmagnetic field may be applied to a fiber 250, 350, 550 in place of themodulation provided by the vibrating piston 240, 340, 540. A detector230, 330, 530 would therefore be adapted to detect the changes in thepolarization of a propagating optical signal scattered out of the fiber250, 350, 550. For example in one embodiment of the present invention, adetector 230, 330, 530 is equipped with a polarizer(s) to enable thedetector to detect the polarization of the modulated propagating opticalsignal. In addition, the detection would be configured to occur at thefrequency of the time-varying magnetic field. Even further, in alternateembodiments of the present invention, similar methods and apparatusesare implemented for identifying an optical fiber using other types oftime-varying birefringence, such as squeezing, etc. These embodiments ofthe present invention have the advantage that the time-varying“signature” may be introduced with very little loss such that the powerand frequency of a propagating optical signal in the fiber remainssubstantially the same and only the direction of polarization changes.

It should be noted however, that with this approach if a propagatingoptical signal, while entering the region of the fiber having thetime-varying birefringence of the present invention, has the same stateof polarization as the birefringence (i.e. it is in an eigenstate of thebirefringence), the optical signal merely experiences a change invelocity but does not undergo a change in its state of polarization.Thus, a fiber identification device configured to detect a change inpolarization would fail to detect any change. As such, it is importantto ensure that the birefringence applied does not have the same state ofpolarization as a propagating optical signal (i.e., the fiber may bebent or twisted, or fixed forms may be used to create a polarizationcontroller).

Although various specific methods of imparting a time-varying modulationon a propagating optical signal are presented herein within specificembodiments of the present invention, it will be appreciated by thoseskilled in the art informed by the teachings of the present inventionthat substantially any method for imparted a time-varying modulation ona propagating optical signal in a fiber may be used to subsequentlyidentify the fiber in accordance with the present invention. Forexample, there are a number of non-linear interactions that may shiftthe frequency of light, and such interactions may be used to impose aknown frequency shift on a propagating optical signal in a fiber. Morespecifically, several optical sensor and signal processing techniquesimplement the interaction of acoustic waves and light to impart afrequency shift. That is, the light interacts with the acoustic phononsto create light at upshifted or downshifted frequencies. Such shifts infrequency may be subsequently detected to identify an optical fiber inaccordance with the present invention. Such frequency shifts may beimparted by various means, such as acoustic horns or acoustictransducers. In addition, other means, such as a lithium-niobate phasemodulator, may be implemented to produce such non-linear interactions.

The present invention may also be used to establish logical continuitythrough devices by separating the transmit head and the receive head.For example, FIG. 6 depicts a high level block diagram of a passiveoptical network (PON). The PON 600 of FIG. 6 comprises an input branch602, a trunk 605 and a plurality of output branches 610 ₁-610 _(N)(collectively output branches 610). The PON 600 of FIG. 6 furthercomprises a fiber identification device comprising a transmitter head640 and receiver head 625 in accordance with the present invention. Inthe PON 600 of FIG. 6, light from the trunk 605 is divided into theoutput branches 610 and transmitted to subscribing customers (notshown). Signals (i.e., in the form of light) from the customers iscombined in the trunk 605 and transmitted back to a head end (notshown). Continuity of communication between the head end and thecustomers may be verified in either direction implementing the conceptsof the present invention in much the same manner as described above. Forexample, regarding point X which is on the input branch 602, and point Ywhich is on one of the branches, the transmit head 640 may be locatednear point X and the receive head 625 on point Y to establish that lightwas traveling from X to Y or vice versa. This might be useful, forinstance, in a situation in which the fiber Y were one of a multiplicityof fibers in a closet, all carrying traffic, that might be otherwiseindistinguishable. As such, communication may be verified without theneed to interrupt the operation of the PON 600.

While the forgoing is directed to various embodiments of the presentinvention, other and further embodiments of the invention may be devisedwithout departing from the basic scope thereof. As such, the appropriatescope of the invention is to be determined according to the claims,which follow.

What is claimed is:
 1. An apparatus for detecting a presence of atime-varying modulation in an optical fiber, the apparatus comprising: afirst modulating device that facilitates imparting the time-varyingmodulation onto an existing optical signal propagating in the opticalfiber; a first fiber bending device that facilitates bending the opticalfiber such that a portion of the optical signal is scattered out of theoptical fiber; a first detector for receiving the portion of the opticalsignal that is scattered for detecting the presence of the time-varyingmodulation; and a lightguide that facilitates guiding the portion of theoptical signal that is scattered out of the optical fiber to the firstdetector.
 2. The apparatus of claim 1, wherein the imparting anddetecting do not interrupt a propagation of the optical signal along theoptical fiber.
 3. The apparatus of claim 1, wherein the optical fibercomprises a plurality of interconnected optical fibers.
 4. The apparatusof claim 1, further comprising a control unit, where the control unitcomprising a memory for storing information and program instructions anda processor for executing the program instructions to configure theapparatus to control: the imparting the time-varying modulation onto theoptical signal propagating in the optical fiber; and the detecting thepresence of the imparted time-varying modulation to identify the opticalfiber.
 5. The apparatus of claim 4, wherein the control unit is furtherconfigured to cause a source of the optical signal to impart thetime-varying modulation onto the optical signal.
 6. The apparatus ofclaim 1, wherein the first modulating device comprises a transmitterhead, and the first fiber bending device and the first detector comprisea receiver head.
 7. The apparatus of claim 6, wherein the transmitterhead further comprises a second fiber bending device and a seconddetector, and the receiver head further comprises a second modulatingdevice.
 8. The apparatus of claim 1, wherein the first modulating devicecomprises a vibrating piston and the vibrating piston varies a curvatureof a portion of the optical fiber as a function of time such that atime-varying loss of power is generated in the propagating opticalsignal.
 9. The apparatus of claim 1, wherein the first modulating devicecomprises a piezo-electric transducer and the piezo-electric transducervaries a curvature of a portion of the optical fiber as a function oftime such that a time-varying loss of power is generated in thepropagating optical signal.
 10. The apparatus of claim 1, wherein thefirst fiber bending device is adjustable for varying a radius of a bendon the optical fiber.
 11. The apparatus of claim 1, wherein the firstfiber bending device comprises an anvil.
 12. The apparatus of claim 1,wherein the lightguide comprises a plexiglass lightguide.
 13. Theapparatus of claim 1, wherein the first modulating device introduces avarying magnetic field and varies a polarization of the propagatingoptical signal as a function of time by varying a birefringence of theoptical fiber as a function of time.
 14. The apparatus of claim 13,wherein the first modulating device comprises a solenoid.
 15. Theapparatus of claim 13, wherein the first detector further comprises apolarizer.
 16. The apparatus of claim 1, wherein the first modulatingdevice varies a frequency of the propagating optical signal as afunction of time through non-linear interactions.
 17. The apparatus ofclaim 16, wherein the first modulating device comprises an acoustichorn.
 18. The apparatus of claim 1, further comprising at least a seconddetector for detecting the time-varying modulation near a point ofmodulation such that a subsequent downstream detection of the modulationis compared to the modulation detected near the point of modulation foridentification of the optical fiber.
 19. The apparatus of claim 1,wherein the apparatus is implemented to verify communications between atleast two points in a passive optical network.
 20. The apparatus ofclaim 1, wherein the apparatus is implemented to verify communicationsbetween a plurality of points in a passive optical network.